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How do I activate XP using Windows 7 Product Key for downgrade? (For Boot Camp on a Mac)

I was told by two computer stores that I can still install Windows XP Pro if I purchase Windows 7 Premium and use the Product Key to activate XP. But I am at the point in the XP Pro installation and the Win 7 PK doesn't work and to my real surprise the XP PK doesn't work.

What are the proper steps to install XP as a downgrade of Win 7? And any ideas why the PK for Win XP Pro (OEM) won't work and let me finish the installation?

Thanks.

PS: I am doing this on a MacBook Pro as part of a Boot Camp 3.0 installation. But I'm not sure that should matter.
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Most Valuable Expert 2013

Commented:
Downgrade rights to XP from W7 apply only to the Ultimate and Professional versions of W7
Which XP OEM provided the key? Unless it was provided with the Mac then it won't install because
a) Since 2005 the OEM keys from the larger manufacturers and printed on CoA stickers are not used for installation/activation
b) OEM installs are product specific so will look for hardware from the manufacturer and if not found won't activate.

Author

Commented:
I am surprised that I was sold Windows 7 Premium instead of Ultimate or Professional.

The XP Pro OEM didn't come from a manufacturer with hardware, it was purchased from a e-tailer so it could be used to successfully upgrade a Dell that came with XP Home to Pro.

If I don't have the correct version of Windows 7 to use for the downgrade PK, I still want to get XP installed now and I will sort out the Win 7 PK downgrade in the next few days.

How do I get the XP install finished?

Commented:
You have to call Microsoft local in your country to get the downgrade key.
Most Valuable Expert 2013
Commented:
Most of the answers you need are on this page with MS
http://oem.microsoft.com/script/contentpage.aspx?pageid=552846 
 
Not sure any of it will be good news for you.
Most Valuable Expert 2013

Commented:
Can't be done legally with the combination of software media and hardware available

See also http:Q_26259674.html which is dealing with the same problem but from a different perspective.